1. Field of the Invention
This invention relates to a type of pressure modulating valve device for use in a brake system of a wheel type vehicle, and more particularly to a load responsive pressure modulating valve device which is so constructed that a starting pressure for the pressure modulating operation thereof of which an input hydraulic brake pressure is changed to an output hydraulic brake pressure in a predetermined ratio, is controlled by means of a specific valve operated by a partial pressure of the input hydraulic brake pressure when the input hydraulic brake pressure reaches a pre-set value. The device is capable of distributing the hydraulic brake pressure corresponding to a transient weight transfer from rear wheels of the vehicle to front ones caused by the braking operation to front and rear braking means by varying the starting pressure in response to load and deceleration of the vehicle.
2. Description of the Prior Art
In a wheel type vehicle, such as an automobile having wheels disposed at the front and the rear portions separately from each other and having independent braking means operated by a braking device for the front and rear wheels, respectively, it is a common practice that the the braking device is so constructed as to distribute the hydraulic brake pressure generated from a master cylinder thereof to the front and the rear braking means, respectively. In such a vehicle, it has already been found that the load on the front wheels increases, while the load on the rear wheels decreases owing to an inertia force proportional to deceleration of the vehicle upon applying the braking operation in comparison with that in a normal driving state. As a result of this, the braking effect of the front wheels increases, while that of the rear wheels decreases. Therefore, in case of applying the hydraulic brake pressure generated from the master cylinder to the front and rear braking means, it is preferable to equalize a substantial brake effect on the front wheels with that on the rear wheels by increasing the hydraulic brake pressure of the front braking means as compared with that of the rear braking means. For the purpose of accomplishing the above described effect, the hydraulic brake pressure generated from the master cylinder is supplied to the rear braking means directly, while an increased pressure is supplied to the front braking means or the pressure is supplied to the front braking means directly and at the same time a proportionally reduced pressure is supplied to the rear braking means. Brake systems utilizing a proportional pressure reducing valve enabling an effective pressure differential between front and rear wheels have been adapted for use in passenger car vehicles having a relatively low centre of gravity and small load variations. In vehicles having a higher centre of gravity and large load variation, however, it is necessary to consider the weight transfer caused by the load variation, so that it is preferable to vary the starting value of the pressure modulating operation in response to a load which has been detected automatically.
Several attempts have been proposed for accomplishing the above described necessity, all of them are so constructed that a starting value of a proportional pressure reduction valve means is controlled by a balance condition between an operational load of a control spring biasing a movable valve and an input hydraulic brake pressure being applied to the movable valve.
Briefly speaking known systems can be classified into the following three types:
a. According to the first type, change of a distance between a rear wheel axle and a chassis owing to load variation is detected by a linkage inclusive of a spring member.
b. the second type is to convert the above change of distance caused by the variation of vehicle load into a pressure change of a hydraulic brake pressure mechanism. For example, a change in hydraulic pressure of a shock absorber for rear wheels is utilized for the detection of load variation on rear wheels, and
c. the third type has a pressure reducing valve with a valve means having a deceleration detecting means which is to be operated by a predetermined deceleration. In this type, a part of an input hydraulic brake pressure generated from the master cylinder is supplied to the rear braking means through the pressure reducing valve and, at the same time, another part of the input hydraulic brake pressure is supplied to the deceleration detecting means and further fed to a control chamber having a check valve and a spring housing in which a control spring is mounted. Then, the pressurized fluid is enclosed in the control chamber by dint of the check valve and the deceleration detecting means and the position of the spring housing in the control chamber is controlled by the enclosed hydraulic brake pressure. In accordance with the above described construction, a sealingly enclosed hydraulic brake pressure in the control chamber becomes higher along with an increase in vehicle load, and thus an increase in an operational load of the control spring owing to an increase of a displacement of the spring housing shifts the starting point for the pressure modulating operation of the pressure reducing valve toward high pressure zone.
In known pressure reducing valves as set forth above, despite the requirement of high precision for a set load of a control spring, there are many different spaces between a rear wheel axle and a chassis even in same type vehicle, so that dimensional errors upon assembling a linkage together with a proportioning pressure reducing valve tend to take place. Thus, an apparatus according to the first type is difficult in assembling and adjustment and the linkage is subjected the lowering of function owing to icing in winter.
An apparatus according to the second type is complex in its structure and is difficult to mount on the vehicle. Furthermore the apparatus is excessively sensitive to irregular road surface conditions.
In apparatus according to the third type, as the hysteresis at the outlet side generated in a normal brake releasing process becomes large in proportion to an increase in spring constant of the control spring, and the larger the load capacity of the wheels, the larger the difference between respective hydraulic brake pressures at the starting point for the pressure reducing operation at light and heavy loads becomes. Therefore, if such a pressure reducing valve having less hysteresis is used, a larger allowance of displacement for a spring housing is required so that the volume of circuits is unnecessarily.